The present invention relates to a solid state switch, and more particularly, to an electrically isolated solid state relay controllable by a low-voltage control signal and enabling rapid switching of high voltages and currents.
A solid state relay provides isolation between a control circuit and a switched circuit and may replace an electromechanical device such as a reed relay. A typical solid state relay comprises a light source such as a light-emitting diode (LED) optically coupled across an electrically isolating gap to a photovoltaic diode array. The photodiode array is electrically connected to a semiconductor output device such as a metal oxide semiconductor field effect transistor (MOSFET) which, in turn, is connected to enable or disable the flow of current in a circuit that is to be switched. Light from the LED creates a voltage across the photodiode stack and activates the output device. Alternatively, when light from the LED ceases, the voltage across the photodiode stack collapses and the output device is deactivated.
One limitation of solid state relays is the speed of the switching action. This is the result of a capacitance inherent in the output MOSFET. Each time the photodiode stack is actuated, this capacitance must be charged before the output MOSFET can turn on. Similarly, each time the photodiode stack is deactivated, this capacitance must discharge before the output MOSFET can turn off. The charging and discharging of this inherent capacitance inhibits the speed of the switching function. This problem is magnified when high-power circuits must be switched since larger MOSFETs with greater inherent capacitance must be used. Solid state relays are also susceptible to electrical transients.
U.S. Pat. No. 4,390,790 to Rodriguez discloses a relay that includes a photodiode stack directly connected to an output MOSFET. Rodriguez's use of a turn-off transistor to discharge the output MOSFET gate-to-source capacitance provides some improvement in turn-off speed, but provides no transient protection to the relay. In a second embodiment disclosed in the Rodriguez patent, an optically coupled JFET switch is used to couple the switched voltage to the gate of the output MOSFET. In so doing, the output MOSFET charges at a faster rate than it normally would. In this set-up, however, the switching JFET suffers from the same draw-backs as the output MOSFET. That is, the JFET has associated with it a charging capacitance which must be overcome. Further, in order for the JFET to switch the current necessary to quickly charge the gate-to-source capacitance of the output MOSFET, the JFET switch must be biased significantly beyond its threshold turn-on voltage. Relays such as this are also vulnerable to transient propagation between the control and switched circuits. Rodriquez also discloses a relay comprising two output transistors connected in parallel with either the source or drain terminals interconnected so that load voltage can be blocked without regard to its polarity. In addition, the gate terminals interconnected so that the gate signal from the photodiode stack is simultaneously applied to the gates and the operation of the two output transistors is concurrent. While operation of the relay is simple, connecting the output transistors in parallel increases the parasitic capacitance of the relay and slows the relays switching speed. Relays with the gate terminals directly interconnected increase the vulnerability of the output MOSFETs to parasitic effects from a gate-to-gate push-pull oscillation condition when performing rapid switching at high currents.
What is desired, therefore, is a solid state relay enabling rapid switching at high power levels.